[boost] Re: Perfect Forwarding (almost)

From: Anthony Williams (anthony.williamsNOSPAM_at_[hidden])
Date: 2004-02-10 05:10:10

David Abrahams <dave_at_[hidden]> writes:
> Brian McNamara <lorgon_at_[hidden]> writes:
>
>> On Sun, Feb 08, 2004 at 11:39:51AM -0500, David Abrahams wrote:
>>> Brian McNamara <lorgon_at_[hidden]> writes:
>>> > David Abrahams writes: 
>>> >> 12.2/1 says that the act of binding an rvalue to a (const) reference
>>> >> creates a temporary (which may be elided).  The creation of the
>>> >> temporary requires the existence of a copy ctor with a const&
>>> >> argument.  As far as I know, that rule serves no useful purpose, but I
>>> >> may have missed something.  
>>> >
>>> > I am not sure what portion of the rule you don't like.  Is it the fact
>>> > that they point to a particular copy constructor (the const& one),
>>> > rather than just any available/visible/something copy constructor?
>>> 
>>> No, it's the fact that any additional temporary at all (and thus, a
>>> copy ctor) is needed.
>>> 
>>> > In test10, what do you 'want' to happen here:
>>> >     sink2( <a temporary object, that is, an rvalue> );
>>> 
>>> It should bind a const reference to the temporary without complaining
>>> that the temporary can't be copied.
>>
>> I see.  What about
>>
>>    const X& x = source();    // 12.2/5
>>
>> ?  Or something.  
>
> That's exactly the case I'm talking about.
>
>> I am trying to find the 'hard' cases where the
>> compiler might not be able to easily make the temporary have the right
>> 'lifetime', and thus require an extra copy.
>
> I don't see any problem here.  There's no problem getting the rvalue
> out of source(), or sink(source()) would fail.  Furthermore, no
> compiler I've ever seen will actually make the allowed temporary in
> that case.

How about

struct X{};

struct Y
{
    std::string s;
    X x;
    std::vector<int> v;
};

Y ySource();

const X& x=ySource().x;

This strikes me as more likely to cause a copy than a straight-forward binding
from an X temporary.

Anthony
-- 
Anthony Williams
Senior Software Engineer, Beran Instruments Ltd.
Remove NOSPAM when replying, for timely response.